The vacuum countergravity casting process using a gas permeable mold is described in such prior art patents as the Chandley et al U.S. Pat. No. 4,340,108 issued July 20, 1982 and U.S. Pat. No. 4,606,396 issued Aug. 17,1986. These patents also illustrate apparatus to practice such process.
Typically, the vacuum countergravity casting apparatus includes a mold having a porous, gas permeable upper mold member and a lower mold member secured together, a vacuum chamber around the upper mold member, and means for submerging the bottom of the lower mold member in a pool of molten metal while evacuating the chamber to draw molten metal up into mold cavities in the upper mold member through ingates in the bottom of the lower mold member. As described in aforementioned U.S. Pat. No. 4,340,108, the upper and lower mold members may comprise gas permeable, low temperature, resin-bonded sand mold members which are adhesively secured (glued) together along a common horizontal parting plane. During casting when the mold cavities are evacuated, an external pressure differential is established across the vertical dimension of the bonded sand mold as a result of reduced pressure being applied on the upper mold member and atmospheric pressure being transmitted to the lower mold member through the molten metal.
As prior art workers have attempted to increase mold size to cast a greater number of castings per mold, the horizontal mold dimension has been increased relative to the vertical mold dimension. In particular, the ratio of horizontal mold dimension to vertical dimension has been increased to 7 or 8:1 from a previously used ratio of about 4:1. At the higher dimensional ratios, the mold becomes susceptible to damage as a result of the aforementioned pressure differential across the vertical mold dimension. If the mold is not sufficiently strong, it can deflect or even fail from bending stresses developed in the mold and result in production of defective castings or damaging melt run-out into the casting chamber.
In the past, prior art workers have increased the vertical dimension of the mold concomitant with increases in horizontal mold dimension to increase mold strength. However, increasing the vertical mold dimension adds significantly to the cost and weight of the mold.
Even if the vertical dimension of the mold is increased to this end, prior art workers must hope that the adhesively bonded joint between the upper and lower mold members is strong enough to prevent failure at the joint from the pressure differential thereacross, especially if the vacuum chamber is sealed to the lower mold member.
Furthermore, even for smaller mold sizes having for example dimensional ratios of about 4:1, there is a desire on the part of prior art workers to reduce the vertical mold dimension (mold thickness) relative to the horizontal dimension to lower the cost and weight of such molds.
It is an object of the present invention to provide an improved apparatus and method for vacuum countergravity casting wherein bending stresses developed in the mold are substantially reduced in such a way to permit reduction in the vertical mold dimension relative to horizontal mold dimension and use of thinner molds or molds having a greater horizontal to vertical dimensional ratio with reduced risk of damage to the mold. It is a further object of the invention to provide an improved countergravity casting apparatus and method wherein the upper and lower mold members need not be glued or adhered together.